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A series of Pt/Ce x Zr 1-x O 2-δ (x = 0.4–0.6) solids were synthesized and evaluated for the SCR of NO under lean burn conditions (2.5 vol% O 2 ) using C 3 H 6 and H 2 as reducing agents. SSITKA-Mass Spectrometry, SSITKA-DRIFTS and other in situ DRIFTS experiments were conducted for the first time to gather fundamental information in explaining the remarkable H 2 /C 3 H 6 synergy effect towards steady-state selective reduction of NO into N 2 at T > 400 °C. In particular, the chemical structure of adsorbed active and inactive (spectator) NO x species formed under C 3 H 6 -SCR, H 2 -SCR and H 2 /C 3 H 6 -SCR of NO and the surface coverage and site formation of active NO x were probed. The Pt/Ce 1-x Zr x O 2-δ catalysts present significant differences in their H 2 -SCR performance (NO conversion and N 2 -selectivity) in the low-temperature range of 120–180 °C but practically the same catalytic behavior at higher temperatures. It was proved that the active NO x of the H 2 -SCR path reside within a reactive zone around each Pt nanoparticle which extends to less than one lattice constant within the support surface. The chemical structure of the active intermediate was proved to be the chelating nitrite , whereas nitrosyl, monodentate and bidentate nitrates were considered as inactive species (spectators). It was illustrated for the first time that the presence of 15 vol% H 2 O in the H 2 -SCR feed stream applied over the 0.1 wt% Pt/Ce 0.5 Zr 0.5 O 2 catalyst results in a 25% decrease in the concentration of active NO x , thus partly explaining the drop in activity observed when compared to the H 2 -SCR in the absence of H 2 O. A remarkable activity and N 2 -selectivity enhancement was observed at T > 400 °C when both H 2 and C 3 H 6 reducing agents were used compared to H 2 -SCR or C 3 H 6 -SCR alone. This synergy effect was explained to arise mainly because of the increase of θ Η by the presence of –CH x species derived from adsorbed propylene decomposition on Pt, which block sites of oxygen chemisorption, and of the increase of surface oxygen vacant sites that promote the formation of a more active chelating nitrite (NO 2 − ) species compared to the case of H 2 -SCR.

The present work reports on a detailed study of the effect of support chemical composition and reaction temperature on the mechanism of the water-gas shift (WGS) reaction over supported-Pt catalysts. The effect of the same parameters on the chemical composition and surface concentration of active reaction intermediates was also determined for the first time, information that allowed elucidating also the site location of these intermediates, e.g., support versus metal or support-metal interface. The abovementioned mechanistic information was rigorously provided by the application of steadystate isotopic transient kinetic analysis (SSTIKA) experiments coupled with mass spectrometry (MS) and DRIFTS techniques, and by other transient isotopic experiments designed. It was found that on Pt/CeO2 a switch of the WGS reaction mechanism from "redox" to a combination of redox and "associative formate with - OH group regeneration" is obtained after increasing the reaction temperature from 473 to 573 K. On the other hand, only the redox mechanism operates on Pt/TiO2 of similar Pt mean particle size. Modification of titania support by the deposition of small crystallites of ceria in its pore structure enhanced significantly the activity of WGS at T > 573 K compared to Pt/TiO2, whereas the operated mechanism resembles that occurring on Pt/CeO2. In all supported-Pt catalysts investigated, the concentration of active "carbon-containing" intermediates found in the "carbon-path" was significantly lower than that found for the "hydrogen-containing" intermediates present in the "hydrogen-path", the latter being labile OH and H species formed on the support, and which were found to depend on the support chemical composition and reaction temperature. This study provides new important fundamental knowledge on the mechanism of WGS over practical supported metal catalysts which can be used for a better WGS catalyst design. © 2011 American Chemical Society. 115 23 11595 11610 Cited By :60

A series of x wt.% Pt/CeO2 catalysts (x = 0.1-2.0) was prepared to investigate for the first time the effect of Pt particle size on important kinetic and mechanistic aspects of the water-gas shift (WGS) reaction, namely the concentration (μmol g-1) and chemical structure of active adsorbed reaction intermediates present in the "carbon-path" and "hydrogen-path" of the WGS reaction at 300 °C. For this, steady-state isotopic transient kinetic analysis (SSITKA) coupled with in situ DRIFTS and mass spectrometry experiments was performed using D2O and 13CO. α novel transient isotopic experiment performed allowed to quantify the initial transient rates of reactions of adsorbed formate (-COOH) and CO by water, based on which it was concluded that formate should not be considered as an important intermediate. According to the present work, it is proposed that the WGS reaction on ceria-supported Pt at 300 °C occurs largely via the "redox" mechanism, and to a lesser extent via the "associative formate with -OH group regeneration" mechanism. The TOF (s-1) of WGS was found to vary only slightly with Pt particle size (1.3-8.0 nm), while the specific reaction rate based on the length of periphery of Pt-CeO2 interface (μmol cm-1 s-1) was found to significantly increase with increasing platinum loading and mean particle size in the 250-300 °C range. © 2011 Elsevier Inc. All rights reserved. 279 2 287 300 Cited By :110

A detailed kinetic and mechanistic study of the water-gas shift (WGS) reaction on a 0.5 wt% Pt/TiO2 catalyst has been carried out. The dependence of kinetic reaction rate on the partial pressures of reactants (CO, H2O) and products (H2, CO2), and the concentration and chemical structure of active and inactive reaction intermediates that are found in the "hydrogen-path" and "carbon-path" of the reaction have been investigated in the 200-270 °C range. The most likely mechanistic pathway of the WGS reaction over the present catalytic system is discussed. It has been found that the reaction rate increases with an increase in the concentration of CO or H2O in the feed stream, while it decreases significantly with the addition of H2 in the feed stream. On the contrary, the kinetic reaction rate was found to be practically independent on the concentration of CO2 in the feed stream. The experimental reaction rates that were estimated were fitted to an empirical power-law rate expression from which the kinetic reaction orders with respect to CO, H2O, CO2, and H2 were estimated to be 0.5, 1.0, ∼0.0, and -0.7, respectively. An apparent activation energy of 10.8 kcal/mol was also estimated. The formation of formate and carbonate surface species over the TiO2 support under WGS reaction conditions was proved via SSITKA-DRIFTS experiments. However, these reaction intermediates must be considered as inactive (spectator) species for the steady-state WGS reaction. Additional transient experiments that involved 18O-isotope provided strong support for the red-ox mechanism as the prevailing one on the present Pt/TiO2 catalyst, where labile oxygen and oxygen vacancies of TiO2 near the metal-support interface can participate in the reaction path of the WGS reaction. © 2009 Elsevier Ltd. All rights reserved. 264 2 117 129 Cited By :106

ID: 1176 In: Catalysis science & technology, Vol. 5, no. 8 (Aug. 2015), p.4108-4115. Summary: AbstractA series of multimetallic copper(ii) complexes have been re-investigated for methane oxidation with H2O2.

The selective catalytic reduction of NOx by H2 (H2-SCR) under strongly oxidizing conditions (520 ppm NOx/1% H2/5% O2/10% CO2/He; NO:NO2–4:1–9:1) in the 150–600 °C range has been studied over 3 wt-% W-promoted CeO2–ZrO2 solids (85 wt-% CeO2-15 wt-% ZrO2 (CeZr), and 17 wt-% CeO2–83 wt-% Zr (ZrCe) synthesised by a proprietary method) for the first time. The highest NOx conversion ( X NO x = 54 % ) was obtained on the W-ZrCe (Zr-rich) solid at 300 °C (GHSV of 51,000 h−1), whereas N2-selectivity was in the 77–92%-range over both W-ZrCe (Zr-rich) and W-CeZr (Ce-rich) catalysts. Significantly higher integral specific rates (RNO, μmol NO m−2 min−1) were estimated on the W-ZrCe (Zr-rich) catalyst compared to the W-CeZr (Ce-rich) one in the 250–350 °C range. The formation of adsorbed NOx under 0.1% NO/10% O2/He gas treatment at 25 °C followed by H2/O2-TPSR experiments revealed that at least two different kinds of active NOx of low concentration (4–7 μmol g−1) were formed on both catalysts, whereas other inactive (spectator) NOx species formed were of larger concentration (>160 μmol g−1). UV–vis/DRS studies revealed that deposition of 3 wt-% W on ZrCe (Zr-rich) mixed metal oxide following calcination at 600 °C resulted in the formation of both polymeric WOx and WO3 clusters, whereas on CeZr (Ce-rich) only the latter phase (W6+) was seen. Large differences in the concentration (μmol m−2) and strength of surface acid sites between the W-CeZr and W-ZrCe solids were revealed after performing NH3-TPD and NH3-DRIFTS. These results were found to correlate with the specific H2-SCR rate (μmol m−2 min) obtained for the two solids. In particular, the surface acid sites on W-ZrCe and W-CeZr solids were found to be 5.96 and 2.76 μmol m−2, respectively, whereas the specific reaction rate was 0.14 and 0.046 μmol m−2 min at 300 and 250 °C, at which maximum rates were observed, respectively.

Platinum nanoparticles ( d Pt = 1.0–1.2 nm) supported on single CeO 2 and La 2 O 3 metal oxides and Ce 0.8 La 0.2 O 2− δ solid solution were prepared to investigate for the first time the effect of La 3+ -doping of ceria on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS at 250 and 300 °C, (ii) the chemical nature of inactive species formed during WGS, and (iii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. For this, steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry were performed to follow the H-path (use of D 2 O) and C-path (use of 13 CO) of the WGS. In addition, other transient isotopic experiments using operando methodology (use of DRFTS and mass spectrometry) were designed to follow with time on stream the reactivity toward water of the various adsorbed species formed under WGS. It is proposed that on Pt/Ce 1− x La x O 2− δ ( x = 0.0, 0.2 and 1.0) the WGS reaction follows both the “redox” and “associative” mechanisms but the extent of participation of each mechanism to the overall WGS reaction rate depends on the support chemical composition. The WGS kinetic rate (μmol CO g −1 s −1 ) increased by a factor of 2.0 and 2.8 at 300 °C on 0.5 wt% Pt supported on Ce 0.8 La 0.2 O 2− δ compared to CeO 2 and La 2 O 3 , respectively. This was explained by (i) the larger concentration of active surface intermediates formed around each Pt nanoparticle (larger extent of reactive zone) and (ii) the higher reactivity of sites ( k , s −1 ) responsible for CO 2 and H 2 formation on Pt/Ce 0.8 La 0.2 O 2− δ compared to Pt/CeO 2 and Pt/La 2 O 3 . Active OH groups is suggested to be formed on defect sites (Ce 3+ □ s ) of Ce 0.8 La 0.2 O 2− δ as a consequence of the introduction of La 3+ into the ceria lattice, the latter enhancing the concentration of labile oxygen and its surface mobility, important characteristics of the “redox” mechanism.

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry (operando) were performed for the first time to study essential mechanistic aspects of the water-gas shift reaction (WGSR) over a 0.5 wt.% Pt/γ-Al2O3 catalyst. The chemical structure of active and inactive reaction intermediate species present in the carbon-path ("C-path") from CO to the CO2 product gas (use of 13CO) and in the hydrogen-path ("H-path") from H2O to the H2 product gas (use of D2O) of the reaction mechanism were determined. In addition, the concentrations (μmol/g) of active species in both the "C-path" and "H-path" of the WGSR at 350 °C were measured. Based on the large concentration of active species present in the "H-path" (OH/H located on the alumina support), the latter being larger than 28 equivalent monolayers of the exposed Pt metal surface, the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol/g) of active species present in the "C-path" (adsorbed CO on Pt and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support surface towards Pt catalytic sites present in the "H-path" of reaction (back-spillover process) might be considered as a slow reaction step. The latter process was evidenced after conducting the WGS reaction (CO/H2O) in a partially deuterated alumina surface (Pt/γ-Al2O3). At least two kinds of formate (-COOH) species residing on the alumina surface have been identified, one of which was active and leads to the formation of CO2(g) and H2(g), whereas the other kind(s) is/are considered as inactive (spectator) adsorbed reaction intermediate species. © 2008 Elsevier B.V. All rights reserved. 138 3-4 228 234 Cited By :46

Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with mass spectrometry were performed for the first time to study essential mechanistic aspects of the water-gas shift (WGS) reaction over alumina-supported Pt, Pd, and Rh catalysts. In particular, the concentrations (μmol g-1) of active intermediate species found in the carbon-path from CO to the CO2 product gas (use of 13CO), and in the hydrogen-path from H2O to the H2 product gas (use of D2O) of the reaction mechanism were determined. It was found that by increasing the reaction temperature from 350 to 500 °C the concentration of active species in both the carbon-path and hydrogen-path increased significantly. Based on the large concentration of active species present in the hydrogen-path (OH/H located on the alumina support), the latter being larger than six equivalent monolayers based on the exposed noble metal surface area (θ > 6.0), the small concentration of OH groups along the periphery of metal-support interface, and the significantly smaller concentration (μmol g-1) of active species present in the carbon-path (adsorbed CO on the noble metal and COOH species on the alumina support and/or the metal-support interface), it might be suggested that diffusion of OH/H species on the alumina support towards catalytic sites present in the hydrogen-path of reaction mechanism might be considered as a slow reaction step. The formation of labile OH/H species is the result of dissociative chemisorption of water on the alumina support, where the role of noble metal is to activate the CO chemisorption and likely to promote formate decomposition into CO2 and H2 products. It was found that there is a good correlation between the surface concentration and binding energy of CO on the noble metal (Pt, Pd or Rh) with the activity of alumina-supported noble metal towards the WGS reaction. © 2007 Elsevier B.V. All rights reserved. 127 1-4 304 318 Cited By :79